1. Field of the Invention
The present invention relates to a light-emitting device assembly and, more particularly, to a light-emitting device assembly comprising a row of insulating substrates on which light-emitting devices are arranged in line to obtain a light-emitting region of desired size. The invention also relates to a method of fabricating such a light-emitting device assembly.
2. Description of the Prior Art
A known light-emitting device assembly is disclosed in Japanese Patent Publication No. 42188/1991 (corresponding to U.S. Pat. No. 4,536,778). To fabricate this assembly, a plurality of insulating substrates are first prepared. The substrates have desired metallization patterns. On each substrate, an array of light-emitting devices and driver ICs for driving the array are installed. The array of light-emitting devices has a plurality of light-emitting devices (LEDs) formed on a common conducting electrode. The substrates are connected to each other so that the light-emitting devices are arranged in line in the direction of the arrangement of the substrates. Thus, the assembly has desired light-emitting regions.
Where this light-emitting device assembly is used as a printhead of the A4 size, for example, the accuracy at which the arrays of the light-emitting devices are arranged linearly must be enhanced, taking account of the linear arrangement of the light-emitting devices, in order to improve the print quality. For this purpose, if the dimensions of the substrates taken in the main scan direction in which the light-emitting devices are scanned are the A4 size, then distortion of the substrates or heat generated by the light-emitting devices produces a deformation, thus deteriorating the accuracy at which the light-emitting device arrays are installed in line. The size referred to hereinafter is taken in the main scan direction. More specifically, the light-emitting device arrays must be arranged in the direction of arrangement of the substrates to a tolerance of about .+-.15 microns. In order to obtain the desired size, the short substrates are connected to each other and arranged in line. In this case, since the light-emitting device arrays are minute, if the end surfaces of the substrates are made to abut against each other to align the arrays, then it is difficult to arrange the light-emitting device arrays with a high linearity. Furthermore, in order to obtain finer print characters, it is necessary that the light-emitting points be spaced only tens of micrometers from each other. Consequently, it has been difficult to bring the outermost light-emitting device in the array on the side of the abutting end close to the opposite light-emitting device in the other array such that the equal spacing between the light-emitting devices is maintained. To circumvent this problem, the light-emitting device assembly has been fabricated in the manner described below. First, the light-emitting device arrays on the sides of the abutting ends are mounted in such a way that the arrays protrude from the end surfaces of the substrates. The protruding end surfaces of the arrays are made to bear against each other. The substrates are placed in such a manner that the arrays are arranged in line. The substrates are then coupled together with connecting members. As a consequence, the arrays are arranged linearly. The light-emitting spots are uniformly spaced from each other. In this way, the light-emitting device assembly having the desired size is obtained.
In recent years, as technology for fabricating substrates evolves, fabrication of a relatively large (approximately the A4 size) unitary substrate has been made feasible. However, as larger printheads are used, larger substrates (such as A1 or A0 size) have been required. Therefore, the prior art method relying on even such relatively large substrates has limitations. Furthermore, the cost of fabricating these large substrates presents problems. For example, in order to obtain a light-emitting device assembly of the A1 size, four light-emitting device installation substrates of the A4 size must be connected to each other in line. Concomitantly, the technique for mounting and placing light-emitting devices across the junctions of the substrate is still necessary.